The method for manufacturing SOI wafers in which ions-implanted wafers are separated after bonding, i.e., the ion implantation delamination method (an art also referred to as the Smart Cut method (registered trademark)), has attracted attention as a method for manufacturing bonded wafers, particularly a method for manufacturing thin SOI wafers that enables the performance improvements of advanced integrated circuits.
The ion implantation delamination method is an art (See Patent Document 1) to form an SOI wafer in the following manner: An oxide film is formed on at least one of two silicon wafers, and gas ions, such as hydrogen ions or rare gas ions, are implanted from a front surface of one of the silicon wafers (a bond wafer) to form a micro bubble layer (an enclosed layer) in the interior of the wafer. The surface from which the ions are implanted is then brought into close contact with and bonded to the other silicon wafer (a base wafer) through the oxide film, and a heat treatment (a delamination heat treatment) is then performed to cleave one of the wafers (the bond wafer) along the micro bubble layer so that the bond wafer is delaminated into a thin film. A heat treatment (a bonding heat treatment) is then performed to strengthen a bond between the wafers. At this point, the cleave surface (the separation surface) is a surface of an SOI layer and an SOI wafer having a thin SOI film with high uniformity is relatively readily obtained. In the method, the bond wafer and the base wafer may be bonded directly without the oxide film.
In the ion implantation delamination method, the bonding heat treatment is generally performed after separation. It is known that increasing the bonding strength before occurrence of separation with the wafers being in close contact exerts a positive effect on quality, so some methods for increasing the bonding strength have been considered. One of the methods is to plasma-activate a bonded surface before bonding. Examples of the procedure for performing a plasma activation treatment include an open-type atmospheric pressure plasma method in which a wafer disposed on a stage is scanned in its plane with a plasma generating electrode under the atmosphere, and a reduced pressure plasma method in which a wafer is disposed on a stage in a chamber, and plasma is generated with a plate-shaped plasma generating electrodes that are oppositely disposed while the pressure in the chamber is reduced to control the pressure of nitrogen or oxygen (See Patent Documents 2 to 4, for example).
Performing a plasma activation treatment removes surface impurities, such as organic substances, attached to the wafer surface, thereby exposing a clean surface. In addition, it is understood that since a hydroxyl group is easy to bond to an Si dangling bond on the exposed clean surface, the wafer bonding strength in the state where the wafers are in close contact is increased. It is generally known that in plasma activation for bonding, the intensity of plasma needs to be low to the extent that the substances attached to the surface are removed because if the intensity of plasma is too high, damage to the surface occurs, thereby rather decreasing the bonding strength.